allow passing a header offset into annotate-snippets

as shown in the snapshot changes, this allows us to manually align what
annotate-snippets calls the header sigil (the `-->` arrow in the diagnostic
header) with our diff line number separators.

these were aligned automatically in the linter because we were also emitting
snippets with annotate-snippets with the same line numbers, but since the
formatter diagnostics only have diffs and not snippets, the default line number
width was zero.

note that this still isn't perfect because we align with the highest line number
in the entire file, not necessarily the highest _rendered_ line number, as shown
in the updated notebook snapshot. still, it should get us closer to aligned than
not having an offset at all and also end up being correct in most(?) cases.

crates/ruff_annotate_snippets/src/renderer/display_list.rs

@@ -56,6 +56,7 @@ pub(crate) struct DisplayList<'a> {
     pub(crate) stylesheet: &'a Stylesheet,
     pub(crate) anonymized_line_numbers: bool,
     pub(crate) cut_indicator: &'static str,
+    pub(crate) lineno_offset: usize,
 }
 
 impl PartialEq for DisplayList<'_> {
@@ -81,13 +82,14 @@ impl Display for DisplayList<'_> {
                 _ => max,
             })
         });
-        let lineno_width = if lineno_width == 0 {
-            lineno_width
-        } else if self.anonymized_line_numbers {
-            ANONYMIZED_LINE_NUM.len()
-        } else {
-            ((lineno_width as f64).log10().floor() as usize) + 1
-        };
+        let lineno_width = self.lineno_offset
+            + if lineno_width == 0 {
+                lineno_width
+            } else if self.anonymized_line_numbers {
+                ANONYMIZED_LINE_NUM.len()
+            } else {
+                ((lineno_width as f64).log10().floor() as usize) + 1
+            };
 
         let multiline_depth = self.body.iter().fold(0, |max, set| {
             set.display_lines.iter().fold(max, |max2, line| match line {
@@ -124,6 +126,7 @@ impl<'a> DisplayList<'a> {
         term_width: usize,
         cut_indicator: &'static str,
     ) -> DisplayList<'a> {
+        let lineno_offset = message.lineno_offset;
         let body = format_message(
             message,
             term_width,
@@ -137,6 +140,7 @@ impl<'a> DisplayList<'a> {
             stylesheet,
             anonymized_line_numbers,
             cut_indicator,
+            lineno_offset,
         }
     }
 
@@ -1088,6 +1092,7 @@ fn format_message<'m>(
         footer,
         snippets,
         is_fixable,
+        lineno_offset: _,
     } = message;
 
     let mut sets = vec![];

crates/ruff_annotate_snippets/src/snippet.rs

@@ -23,6 +23,7 @@ pub struct Message<'a> {
     pub(crate) snippets: Vec<Snippet<'a>>,
     pub(crate) footer: Vec<Message<'a>>,
     pub(crate) is_fixable: bool,
+    pub(crate) lineno_offset: usize,
 }
 
 impl<'a> Message<'a> {
@@ -59,6 +60,16 @@ impl<'a> Message<'a> {
         self.is_fixable = yes;
         self
     }
+
+    /// Add an offset used for aligning the header sigil (`-->`) with the line number separators.
+    ///
+    /// For normal diagnostics this is computed automatically based on the lines to be rendered.
+    /// This is intended only for use in the formatter, where we don't render a snippet directly but
+    /// still want the header to align with the diff.
+    pub fn lineno_offset(mut self, offset: usize) -> Self {
+        self.lineno_offset = offset;
+        self
+    }
 }
 
 /// Structure containing the slice of text to be annotated and
@@ -173,6 +184,7 @@ impl Level {
             snippets: vec![],
             footer: vec![],
             is_fixable: false,
+            lineno_offset: 0,
         }
     }